In recent years, with the development of digital communications, wireless communication devices, such as cellular phones and smartphones, perform data transmission with high efficiency. When a data-transmitting-side wireless communication device applies a multi-phase modulation method as a data transmitting method, it is important to linearize amplification characteristics of a transmission power amplifier to reduce non-linear distortion, thereby reducing power leaked to an adjacent channel.
Furthermore, to improve power efficiency with use of an amplifier having poor linearity, a technology to compensate non-linear distortion caused by the poor linearity is desirable. Therefore, a distortion compensating unit that compensates non-linear distortion of a power amplifier is provided in a wireless communication device.
The distortion compensating unit compares, for example, a transmission signal converted into I and Q signals by an S/P converter with a feedback signal that a portion of the amplified I and Q signals is fed back, and calculates a distortion compensation coefficient which reduces a difference between the transmission signal and the feedback signal to zero. Then, the distortion compensating unit multiplies the transmission signal converted into the I and Q signals by the distortion compensation coefficient, thereby compensating the distortion. The distortion compensating unit compares the transmission signal before the distortion compensation with a feedback signal that a portion of the signal amplified after the distortion compensation is fed back, and updates the distortion compensation coefficient so that a difference between these signals becomes zero as needed.
In general, non-linear characteristics of a power amplifier is produced in a saturation region. Namely, when power of a signal input to the power amplifier, i.e., the amplitude of a transmission signal is high, distortion is likely to be produced. Therefore, it is important to optimize a distortion compensation coefficient corresponding to high power. However, the appearance frequency of a high-power signal is low, so it takes time to optimize the distortion compensation coefficient. And, while the distortion compensation coefficient is being optimized, a spurious is produced in a band corresponding to an adjacent channel.
For example, as illustrated in FIG. 1, a table associating multiple addresses with distortion compensation coefficients corresponding to the addresses (hereinafter, sometimes also referred to as a “look-up table (LUT)”) is provided. In FIG. 1, the horizontal axis indicates an address, and the vertical axis indicates a value of a distortion compensation coefficient. Here, an address corresponds to power of an input signal. As illustrated in FIG. 1, in the course of the optimization, a difference between an ideal distortion compensation coefficient and an actual distortion compensation coefficient is produced in a high address value. Due to this difference, a spurious is produced.
To solve this problem, conventionally, there has been proposed a technology to fix a distortion compensation coefficient corresponding to an address higher than a certain threshold (hereinafter, sometimes also referred to as “address clip”). In this conventional address clip, a distortion compensation coefficient corresponding to an address higher than the certain threshold (hereinafter, sometimes also referred to as the “clip threshold” or “address clip value”) is set as a distortion compensation coefficient corresponding to the nearest address in addresses equal to or lower than the clip threshold (hereinafter, sometimes also referred to as a “clip coefficient”).
Related-art examples are described, for example, in Japanese Laid-open Patent Publication No. 2011-199428 and Japanese Laid-open Patent Publication No. 2001-284976.
However, when the clip threshold is fixed to a too low value, the address clip is applied also to an address which is higher than the clip threshold and a distortion compensation coefficient corresponding thereto has been optimized. Furthermore, when the clip threshold is fixed to a too high value, a distortion compensation coefficient of an address which has not actually been optimized is used as a clip coefficient. As a result, a spurious is produced.